The late Smithian thermal maximum and the Smithian-Spathian event were the two primary drivers of Early Triassic floristic changes at high southern latitudes

The end-Permian event (EPE, c. 252.3–251.9 Ma) resulted in the catastrophic collapse of continental ecosystems, including the extinction of peat-forming Glossopteridales in southern Gondwana. The Sydney Basin, positioned at high southern latitudes (c. 70°S), preserves a detailed Early Triassic continental succession, offering unique insights into past ecosystem destabilisation and recovery dynamics in the wake of the EPE. Here, we present a high-resolution, age-controlled analysis of floral and environmental trends in the region, integrating palynology, geochemistry, and sedimentology.

Palynological data, aided by non-metric multidimensional analysis, revealed distinct floral shifts linked to key climatic events, including the late Smithian thermal maximum (LSTM, c. 250.3–249.6 Ma) and the Smithian-Spathian cooling event (SSE, c. 249.6–249.2 Ma). Carbon isotopic compositions of bulk organic matter (δ¹³C_org) were used to correlate these shifts with global carbon cycle perturbations. Visible and infrared spectroscopy (HyLogger), combined with sedimentological analysis, determined sediment composition and provenance, while X-ray fluorescence (XRF) provided weathering proxies such as the chemical index of alteration.

The palynological record reveals the ebb and flow of several distinct floristic communities during the first five million years following the EPE. Initial post-collapse ecosystems were dominated by peltasperm seed ferns for c. 200,000 years. This ecosystem was supplanted by voltzialean conifers, which thrived for c. 1.5 million years until their decline during the hyperthermal LSTM. The LSTM interval is demarcated in the Sydney Basin as occurring near the end of a major global negative δ¹³C_org excursion c. 250.2 Ma. Weathering indices and sedimentology of this interval evidenced drier conditions were at play, likely connected with the co-occurrent global rise in temperature. Furthermore, stress-tolerant isoëtalean lycophytes, such as Pleuromeia, became dominant, reflected by a prolonged acme of associated spores (e.g., Densoisporites nejburgii). Intriguingly, while pleuromeians proliferated at low latitudes soon after the EPE, their dominance in the Sydney Basin is recorded only during the LSTM c. 1.7 million years later, perhaps signalling another phase of gymnosperm ecosystem decline and a delayed upsurge of stress-tolerant floras at south polar latitudes.

The SSE, recognised by a major global positive δ¹³C_org excursion c. 249.4 Ma, saw the expansion of umkomasialean seed ferns dominated by the iconic Triassic Gondwanan plant Dicroidium. However, the landscape also saw the intermittent emergence of other pleuromeian spores (i.e., Aratrisporites), in addition to fluctuating floristic diversity following the SSE for c. 1.5 million years. Sedimentological data from this interval show widespread Fe-rich mudrocks ("red beds"), likely reflecting better-drained floodplains. HyLogger and XRF data suggest humid, seasonal environments punctuated by drought periods. Although gymnosperm pollen gradually became the most abundant plant microfossil—denoting the long-term reestablishment of gymnospermous forests—the Sydney Basin apparently still lacked the necessary environmental conditions required for coal formation until well into the Middle Triassic.

These findings reveal a staggered, non-linear recovery of high-latitude ecosystems, with alternating dominance of gymnosperm and lycophyte floras persisting until the Early–Middle Triassic boundary (c. 247.2 Ma).